Sacrificial substrates for silicon-carbon composite materials

ABSTRACT

Methods of forming a composite material film can include providing a layer comprising a carbon precursor and silicon particles on a sacrificial substrate. The methods can also include pyrolysing the carbon precursor to convert the precursor into one or more types of carbon phases to form the composite material film, whereby the sacrificial substrate has a char yield of about 10% or less.

BACKGROUND Field

The present disclosure relates generally to silicon-carbon compositematerials. In particular, the present disclosure relates to sacrificialsubstrates for forming silicon-carbon composite materials.Silicon-carbon composite materials can be used in battery electrodes.

Description of the Related Art

A lithium ion battery typically includes a separator and/or electrolytebetween an anode and a cathode. In one class of batteries, theseparator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand anode (e.g., electrodes) to form the battery. Typical electrodesinclude electro-chemically active material layers on electricallyconductive metals (e.g., aluminum and copper). Films can be rolled orcut into pieces which are then layered into stacks. The stacks are ofalternating electro-chemically active materials with the separatorbetween them.

SUMMARY

In certain implementations, a method of forming a composite materialfilm is provided. The method can include providing a layer comprising acarbon precursor and silicon particles on a sacrificial substrate. Themethod can also include pyrolysing the carbon precursor to convert theprecursor into one or more types of carbon phases to form the compositematerial film, whereby the sacrificial substrate has a char yield ofabout 10% or less. For example, the sacrificial substrate can have achar yield of about 7% or less, about 5% or less, about 3% or less,about 1% or less, about 0%, etc.

In various implementations, the sacrificial substrate can comprisepolymethylpentene (PMP), acetal copolymer, acrylonitrile butadienestyrene (ABS), paraffin wax, polyethylene oxide, polyethylene,polypropylene, poly(propylene carbonate), cellulose acetate, or acombination thereof. For example, the sacrificial substrate can comprisepolyethylene, polypropylene, poly(propylene carbonate),polymethylpentene, or a combination thereof.

In some implementations, the layer can include a mixture comprising asolvent. In some instances, the sacrificial substrate can be insolublein the solvent. In some instances, the solvent can compriseN-Methylpyrrolidone (NMP). In some instances, the solvent can comprisewater. In some such instances, the carbon precursor can comprise a watersoluble polymer.

In certain implementations, the method can further comprise drying themixture on the sacrificial substrate prior to pyrolysing, wherein thedried mixture comprises from about 10% to about 30% of the solvent. Insome instances, the method can further comprise forming the driedmixture on the sacrificial substrate into a plurality of dried mixtureson sacrificial substrates, and stacking the plurality prior topyrolysing.

In some implementations, the method can further comprise providing amixture on a first substrate. The mixture can comprise a solvent. Themethod can also comprise drying the mixture and removing the driedmixture prior to providing the dried mixture on the sacrificialsubstrate. In some instances, the dried mixture can comprise from about10% to about 30% of the provided solvent. In some instances, the methodcan further comprise forming the dried mixture into a plurality of driedmixtures. Providing the mixture on the sacrificial substrate cancomprise providing a stack of the plurality of dried mixtures on aplurality of sacrificial substrates.

In various implementations, the composite material film can comprise theone or more types of carbon phases at greater than 0% to about 20% byweight. In various implementations, the composite material film cancomprise the silicon particles at greater than about 50% to about 99% byweight. In some instances, the composite material film can furthercomprise graphite particles. In certain implementations, the compositematerial film can be substantially electrochemically active. In someinstances, pyrolysing can form the composite material film as aself-supported structure.

In certain implementations, a method of forming an electrochemicaldevice is provided. The method can include providing a first electrode,a second electrode, and electrolyte. The first electrode can compriseproviding the composite material film formed by the provided method offorming a composite material film. In various implementations, providingthe first electrode can comprise providing the composite material filmas a self-supported electrode. In some implementations, providing thefirst electrode can comprise providing the composite material film on acurrent collector. In some instances, the first electrode can be ananode, and the second electrode can be a cathode. The electrochemicaldevice can be a battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example method of forming a composite material inaccordance with certain implementations described herein.

FIG. 2 schematically illustrates an example method of forming a materialon a sacrificial substrate.

FIG. 3 schematically illustrates another example method of forming amaterial on a sacrificial substrate.

FIG. 4 schematically illustrates an example method of forming a materialusing sacrificial substrates.

FIG. 5 schematically illustrates another example method of forming amaterial using sacrificial substrates.

FIG. 6 shows a graph of the capacity retention versus cycle index ofelectrochemical cells with electrodes prepared with sacrificialsubstrates using a method as shown in FIG. 4 compared withelectrochemical cells with electrodes prepared without sacrificialsubstrates.

FIG. 7 shows a graph of the capacity retention versus cycle index ofelectrochemical cells with electrodes prepared with sacrificialsubstrates using a method as shown in FIG. 5 compared withelectrochemical cells with electrodes prepared without sacrificialsubstrates.

DETAILED DESCRIPTION

Silicon-carbon composite electrodes can be formed by preparing a slurryof silicon and a carbon precursor material, and coating that slurry on acarrier substrate. The slurry can be dried into a green film (e.g.,dried mixture) and subsequently separated from the carrier substrate.The unsupported green film can be thermally treated to convert thecarbon precursor into carbon. The resulting silicon-carbon compositefilm can be used as a monolithic self-supporting electrode (e.g., asdescribed in U.S. Pat. No. 9,178,208 entitled “Composite Materials forElectrochemical Storage” and U.S. Patent Application Publication No.2014/0170498 entitled “Silicon Particles for Battery Electrodes,” theentireties of which are hereby incorporated by reference), or can belaminated to an adhesive-coated current collector to create the siliconcarbon-composite electrode (e.g., as described in U.S. Pat. No.9,397,338 entitled “Electrodes, Electrochemical Cells, and Methods ofForming Electrodes and Electrochemical Cells” or U.S. Pat. No. 9,583,757entitled “Electrodes, Electrochemical Cells, and Methods of FormingElectrodes and Electrochemical Cells,” each of which is incorporated byreference herein).

The removal of the green film from the carrier substrate may involvespecialized peeling equipment and/or impose certain requirements on thematerials. For example, it is desirable that the coated film does notadhere too strongly to the carrier substrate and that the unsupportedgreen film is robust, yet flexible enough to be handled. In someinstances, the residual solvent in the slurry is removed via vacuumdrying prior to pyrolysis so that individual layers of green film do notbond together (e.g., when stacked on top of one another) during thermaltreatment. At the same time, in order to improve the ability to peel thegreen film from the carrier substrate, it may be useful to retain acertain amount of residual solvent or plasticizer in the green film. Thepresent disclosure describes methods of pyrolysing on sacrificialsubstrates to advantageously improve the ability to separate the filmfrom the underlying substrate and/or to reduce the processing andmaterial restrictions. In various implementations, the sacrificialsubstrate can be thermally decomposed with relatively low char yield(e.g., approximately 10% or lower yield), using the same thermaltreatment conditions that convert the carbon precursor into carbon.

FIG. 1 illustrates an example method of forming a composite material.The method 100 can include providing a layer comprising a carbonprecursor and silicon particles on a sacrificial substrate, block 110.The method 100 can further include pyrolysing the carbon precursor toconvert the precursor into one or more types of carbon phases to formthe composite material film, whereby the sacrificial substrate has achar yield of about 10% or less, block 120.

With reference to block 110, the layer comprising a carbon precursor canbe provided on a substrate as described in U.S. Pat. Nos. 9,178,208,9,397,338, or U.S. Pat. No. 9,583,757. For example, the layer cancomprise a mixture. The mixture can include a variety of differentcomponents. The mixture can include one or more precursors. In certainembodiments, the precursor is a hydrocarbon compound. For example, theprecursor can include polyamic acid, polyimide, etc. Other precursorsinclude phenolic resins, epoxy resins, and other polymers. The mixturecan further include a solvent. For example, the solvent can beN-methyl-pyrollidone (NMP). Other possible solvents include acetone,diethyl ether, gamma butyrolactone, isopropanol, dimethyl carbonate,ethyl carbonate, dimethoxyethane, water, etc. Examples of precursor andsolvent solutions include PI-2611 (HD Microsystems), PI-5878G (HDMicrosystems) and VTEC PI-1388 (RBI, Inc.). PI-2611 is comprised of >60%n-methyl-2-pyrollidone and 10-30%s-biphenyldianhydride/p-phenylenediamine. PI-5878G is comprised of >60%n-methylpyrrolidone, 10-30% polyamic acid of pyromelliticdianhydride/oxydianiline, 10-30% aromatic hydrocarbon (petroleumdistillate) including 5-10% 1,2,4-trimethylbenzene. Another example ofprecursor and solvent solutions is a water soluble polymer in water. Incertain implementations, the amount of precursor (e.g., solid polymer)in the solvent is about 10 wt. % to about 30 wt. %. In general, slurryformulations may contain at least 20 wt % polymer on a solids basis inorder for the dried mixture to maintain sufficient flexibility and avoidcracking and crumbling when removed from a carrier substrate. However,when using sacrificial substrates as described herein, the amount ofpolymer can be reduced. For example, the amount of polymer can be fromabout 2% to about 10% (e.g., about 2%, about 3%, about 4%, about 5%,about 6%, about 7%, about 8%, about 9%, about 10%, etc.) or from anyrange formed by such values. Additional materials can also be includedin the mixture. For example, silicon particles or carbon particlesincluding graphite active material, chopped or milled carbon fiber,carbon nanofibers, carbon nanotubes, and other conductive carbons can beadded to the mixture. In addition, the mixture can be mixed tohomogenize the mixture.

In certain embodiments, the mixture is cast on a carrier substrate(e.g., prior to being provided on a sacrificial substrate). The carriersubstrate can be glass, ceramic, or metal (e.g., aluminum). In someinstances, the carrier substrate can be a polymer, for example,polyethylene terephthalate (PET). In some implementations, castingincludes using a gap extrusion or a blade casting technique. The bladecasting technique can include applying a coating to the substrate byusing a flat surface (e.g., blade) which is controlled to be a certaindistance above the substrate. A liquid or slurry can be applied to thesubstrate, and the blade can be passed over the liquid to spread theliquid over the substrate. The thickness of the coating can becontrolled by the gap between the blade and the substrate since theliquid passes through the gap. As the liquid passes through the gap,excess liquid can also be scraped off. For example, the mixture can becast on a polymer sheet, a polymer roll, or foils or rolls made of glassor metal. The mixture can then be dried to remove at least some of thesolvent. For example, a polyamic acid and NMP solution can be dried atabout 110° C. for about 2 hours to remove the NMP solution. In someinstances, the dried mixture can comprise from about 10% to about 30% ofthe provided solvent. The dried mixture coated on the carrier substratecan form a green film. The dried mixture can then be removed from thesubstrate. For example, an aluminum substrate can be etched away withHCl. Alternatively, the dried mixture can be removed from the substrateby peeling or otherwise mechanically removing the dried mixture from thesubstrate. In certain embodiments, the dried mixture is a precursor filmor sheet. In some embodiments, the dried mixture is optionally cured. Insome implementations, the dried mixture may be further dried. Forexample, the dried mixture can be placed in a hot press (e.g., betweengraphite plates in an oven). A hot press can be used to further dryand/or cure and to keep the dried mixture flat. For example, the driedmixture from a polyamic acid and NMP solution can be hot pressed atabout 200° C. for about 8 to 16 hours. Alternatively, the entire processincluding casting and drying can be done as a roll-to-roll process usingstandard film-handling equipment. The dried mixture can be rinsed toremove any solvents or etchants that may remain. For example, de-ionized(DI) water can be used to rinse the dried mixture. In someimplementations, the mixture can be coated on a substrate by a slot diecoating process (e.g., metering a constant or substantially constantweight and/or volume through a set or substantially set gap). In certainembodiments, tape casting techniques can be used for the casting. Insome other embodiments, there is no substrate for casting and the anodefilm does not need to be removed from any substrate.

The dried mixture can be placed on a sacrificial substrate, where itwill be pyrolysed. In various implementations, the sacrificial substratecan be thermally decomposed with a relatively low char yield using thesame thermal treatment conditions to convert the carbon precursor intocarbon. The sacrificial substrate can be any material that can be formedinto a film and pyrolysed in inert gas with low residue. In someinstances, the sacrificial substrate can have a char yield of about 10%or less (e.g., about 10%, about 9% or less, about 8% or less, about 7%or less, about 6% or less, about 5% or less, about 4% or less, about 3%or less, about 2% or less, about 1% or less, about 0%, etc.). Ingeneral, the char yield can be the percent of solid obtained afterpyrolysis. In some instances, the sacrificial substrate can have acarbon yield of about 10% or less (e.g., about 10%, about 9% or less,about 8% or less, about 7% or less, about 6% or less, about 5% or less,about 4% or less, about 3% or less, about 2% or less, about 1% or less,about 0%, etc.). Example materials for the sacrificial substrate caninclude polyethylene, polypropylene, poly(propylene carbonate), orpolymethylpentene (PMP). Other examples include acetal copolymer,acrylonitrile butadiene styrene (ABS), paraffin wax, polyethylene oxide,or cellulose acetate. Another example is polystyrene, e.g., with waterbased solvents. The sacrificial substrate can also be made of acombination (e.g., two or more) of any of these materials. Othermaterials can also be used.

In some implementations, the sacrificial substrate material can beinsoluble in the solvent used in the slurry. For example, apolypropylene substrate may be suitable for use with a slurry containingNMP. As another example, an ABS substrate might not be appropriate ifthe ABS would dissolve in NMP.

With reference to block 120, the carbon precursor is pyrolysed on thesacrificial substrate. The precursor can be pyrolysed as described inU.S. Pat. Nos. 9,178,208, 9,397,338, or U.S. Pat. No. 9,583,757. Forexample, the mixture further goes through pyrolysis to convert theprecursor to carbon. In certain embodiments, the mixture is pyrolysed ina reducing atmosphere. For example, an inert atmosphere, a vacuum and/orflowing argon, nitrogen, or helium gas can be used. In some embodiments,the mixture is heated to about 900° C. to about 1350° C. For example,polyimide formed from polyamic acid can be carbonized at about 1175° C.for about one hour. In certain embodiments, the heat up rate and/or cooldown rate of the mixture is about 10° C./min. After pyrolysis, the charand/or carbon yield of the sacrificial substrate can be about 10% orless (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0%, etc.). Invarious implementations, the pyrolysed carbon has been easily removedfrom the sacrificial substrate and the remaining low char and/or carbonyield of the sacrificial substrate need not be removed.

In some implementations, instead of providing and drying a mixture on acarrier substrate, removing the green film from the carrier substrate,and providing the green film on a sacrificial substrate, the mixture canbe provided and dried directly on the sacrificial substrate to form abilayer film (e.g., green film and sacrificial substrate). In someinstances, the green film can comprise from about 10% to about 30% ofthe provided solvent. Advantages include reducing the amount ofprocessing and handling steps.

FIG. 2 schematically illustrates an example of the described methods. Inthe method 200, a layer 210 of green film is provided on a sacrificialsubstrate 220. The layer 210 can be provided either indirectly afterbeing removed from a carrier substrate or dried from a mixture provideddirectly on the sacrificial substrate 220. The layer 210 of green filmcan be pyrolysed on the sacrificial substrate 220.

In some instances, a holder/fixture may be used to keep the mixture in aparticular geometry. The holder can be graphite, metal, etc. In certainembodiments, the mixture is held flat. FIG. 3 schematically illustratesan example of the described method. In the method 300, the layer 310 ofgreen film is provided on a sacrificial substrate 320 and held within aholder 330. In some instances, a sacrificial material 340 (e.g., asacrificial separator/interleaf) can be provided between the layer 310of green film and the holder/fixture 330. The sacrificial material 340can have a char and/or carbon yield of about 10% or less (e.g., about10%, about 9% or less, about 8% or less, about 7% or less, about 6% orless, about 5% or less, about 4% or less, about 3% or less, about 2% orless, about 1% or less, about 0%, etc.). In some instances, thesacrificial material 340 can be the same material as the sacrificialsubstrate 320. In some instances, the sacrificial material 340 can be adifferent material than the sacrificial substrate 320. After the greenfilm is pyrolysed, tabs can be attached to the pyrolysed material toform electrical contacts. For example, nickel, copper or alloys thereofcan be used for the tabs.

In some implementations, the green film can be formed into a pluralityof green films. For example, after being removed from a carriersubstrate, the green film may be cut or mechanically sectioned intosmaller pieces. FIG. 4 schematically illustrates an example method 400using a plurality of green films 410 and a plurality of sacrificialsubstrates 420. As shown, the green films 410 can be provided in a stackwith the sacrificial substrates 420. For example, a sacrificialsubstrate can be provided as a separator/interleaf 420 placed in betweentwo layers of the green film 410. A fixture 430 can be used to keep thegreen films 410 and sacrificial substrates 420 together duringpyrolysis. A sacrificial separator/interleaf 420 can also be placed inbetween any green film 410 and the fixture 430. In some instances, thegreen films 410 and sacrificial substrates 420 can also be providedside-by-side and/or in multiple stacks.

FIG. 5 schematically illustrates another example method 500 of forming amaterial using sacrificial substrates. In some implementations, thebilayer film (e.g., green film provided either directly or indirectly ona sacrificial substrate) can be formed into a plurality of bilayerfilms. For example, a bilayer film may be cut or mechanically sectionedinto smaller pieces. As shown in FIG. 5, the bilayer film 510/520 can becut into pieces and stacked within a fixture 530. Although for purposesof illustration, the plurality of bilayer films 510/520 in FIG. 5 isshown with spacing between one another, the bilayer films 510/520 can bestacked on top of one another. A sacrificial separator/interleaf 520 canbe placed in between any green film 510 and the fixture 530. The fixture530 can used to keep the green films 510 and sacrificial substrates 520together during pyrolysis. In some instances, the green films 510 andsacrificial substrates 520 can also be provided side-by-side and/or inmultiple stacks.

In certain embodiments, one or more of the methods described herein is acontinuous process. For example, casting, drying, possible curing andpyrolysis can be performed in a continuous process; e.g., the mixturecan be coated onto a glass or metal cylinder, removed, and placed on asacrificial substrate or the mixture can be coated onto the sacrificialsubstrate. The mixture can be dried while rotating on the cylindercreating a film. The film can be transferred as a roll or peeled and fedinto another machine for further processing (e.g., placed on asacrificial substrate and pyrolysed). Extrusion and other filmmanufacturing techniques known in industry could also be utilized priorto the pyrolysis step.

Pyrolysis of the precursor results in a carbon material (e.g., at leastone carbon phase). In certain embodiments, the carbon material is a hardcarbon. In some embodiments, the precursor is any material that can bepyrolysed to form a hard carbon. When the mixture includes one or moreadditional materials or phases in addition to the carbonized precursor,a composite material can be created. In particular, the mixture caninclude silicon particles creating a silicon-carbon (e.g., at least onefirst phase comprising silicon and at least one second phase comprisingcarbon) or silicon-carbon-carbon (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, and atleast one third phase comprising carbon) composite material. Siliconparticles can increase the specific lithium insertion capacity of thecomposite material. When silicon absorbs lithium ions, it experiences alarge volume increase on the order of 300+ volume percent which cancause electrode structural integrity issues. In addition to volumetricexpansion related problems, silicon is not inherently electricallyconductive, but becomes conductive when it is alloyed with lithium(e.g., lithiation). When silicon de-lithiates, the surface of thesilicon losses electrical conductivity. Furthermore, when siliconde-lithiates, the volume decreases which results in the possibility ofthe silicon particle losing contact with the matrix. The dramatic changein volume also results in mechanical failure of the silicon particlestructure, in turn, causing it to pulverize. Pulverization and loss ofelectrical contact have made it a challenge to use silicon as an activematerial in lithium-ion batteries. A reduction in the initial size ofthe silicon particles can prevent further pulverization of the siliconpowder as well as minimizing the loss of surface electricalconductivity. Furthermore, adding material to the composite that canelastically deform with the change in volume of the silicon particlescan ensure that electrical contact to the surface of the silicon is notlost. For example, the composite material can include carbons such asgraphite which contributes to the ability of the composite to absorbexpansion and which is also capable of intercalating lithium ions addingto the storage capacity of the electrode (e.g., chemically active).Therefore, the composite material may include one or more types ofcarbon phases.

In some embodiments, the particle size (e.g., diameter or a largestdimension) of the silicon particles can be less than about 50 μm, lessthan about 40 μm, less than about 30 μm, less than about 20 μm, lessthan about 10 μm, less than about 1 μm, between about 10 nm and about 50μm, between about 10 nm and about 40 μm, between about 10 nm and about30 μm, between about 10 nm and about 20 μm, between about 0.1 μm andabout 20 μm, between about 0.5 μm and about 20 μm, between about 1 μmand about 20 μm, between about 1 μm and about 15 μm, between about 1 μmand about 10 μm, between about 10 nm and about 10 μm, between about 10nm and about 1 μm, less than about 500 nm, less than about 100 nm, about100 nm, etc. All, substantially all, or at least some of the siliconparticles may comprise the particle size (e.g., diameter or largestdimension) described above. For example, an average particle size (orthe average diameter or the average largest dimension) or a medianparticle size (or the median diameter or the median largest dimension)of the silicon particles can be less than about 50 μm, less than about40 μm, less than about 30 μm, less than about 20 μm, less than about 10μm, less than about 1 μm, between about 10 nm and about 50 μm, betweenabout 10 nm and about 40 μm, between about 10 nm and about 30 μm,between about 10 nm and about 20 μm, between about 0.1 μm and about 20μm, between about 0.5 μm and about 20 μm, between about 1 μm and about20 μm, between about 1 μm and about 15 μm, between about 1 μm and about10 μm, between about 10 nm and about 10 μm, between about 10 nm andabout 1 μm, less than about 500 nm, less than about 100 nm, about 100nm, etc. In some embodiments, the silicon particles may have adistribution of particle sizes. For example, at least about 95%, atleast about 90%, at least about 85%, at least about 80%, at least about70%, or at least about 60% of the particles may have the particle sizedescribed herein.

The amount of silicon provided in the mixture or in the compositematerial can be greater than zero percent by weight of the mixtureand/or composite material. In certain embodiments, the amount of siliconcan be within a range of from about 0% to about 99% by weight of thecomposite material, including greater than about 0% to about 99% byweight, greater than about 0% to about 95% by weight, greater than about0% to about 90%, greater than about 0% to about 35% by weight, greaterthan about 0% to about 25% by weight, from about 10% to about 35% byweight, at least about 30% by weight, from about 30% to about 99% byweight, from about 30% to about 95% by weight, from about 30% to about90% by weight, from about 30% to about 80% by weight, at least about 50%by weight, from about 50% to about 99% by weight, from about 50% toabout 95% by weight, from about 50% to about 90% by weight, from about50% to about 80% by weight, from about 50% to about 70% by weight, atleast about 60% by weight, from about 60% to about 99% by weight, fromabout 60% to about 95% by weight, from about 60% to about 90% by weight,from about 60% to about 80% by weight, at least about 70% by weight,from about 70% to about 99% by weight, from about 70% to about 95% byweight, from about 70% to about 90% by weight, etc. In variousembodiments described herein, the amount of silicon can be 90% orgreater by weight, e.g., about 90% or greater to about 95% by weight,about 90% or greater to about 97% by weight, about 90% or greater toabout 99% by weight, about 92% or greater to about 99% by weight, about95% or greater to about 99% by weight, about 97% or greater to about 99%by weight, etc.

Furthermore, the silicon particles may or may not be pure silicon. Forexample, the silicon particles may be substantially silicon or may be asilicon alloy. In one embodiment, the silicon alloy includes silicon asthe primary constituent along with one or more other elements. Forexample, these elements may include aluminum (Al), iron (Fe), copper(Cu), oxygen (O), or carbon (C).

The amount of carbon obtained from the precursor can be about 50 weightpercent from, e.g., polyamic acid. In certain embodiments, the amount ofcarbon obtained from the precursor in the composite material can begreater than 0% to about 95% by weight such as greater than 0% to about90% by weight, greater than 0% to about 80% by weight, greater than 0%to about 70% by weight, greater than 0% to about 60% by weight, greaterthan 0% to about 50% by weight, greater than 0% to about 40% by weight,greater than 0% to about 30% by weight, greater than 0% to about 20% byweight, about 1% to about 95% by weight, about 1% to about 90% byweight, 1% to about 80% by weight, about 1% to about 70% by weight,about 1% to about 60% by weight, about 1% to about 50% by weight, about1% to about 40% by weight, about 1% to about 30% by weight, about 1% toabout 20% by weight, about 5% to about 95% by weight, about 5% to about90% by weight, about 5% to about 80% by weight, about 5% to about 70% byweight, about 5% to about 60% by weight, about 5% to about 50% byweight, about 5% to about 40% by weight, about 5% to about 30% byweight, about 5% to about 20% by weight, about 10% to about 95% byweight, about 10% to about 90% by weight, about 10% to about 80% byweight, about 10% to about 70% by weight, about 10% to about 60% byweight, about 10% to about 50% by weight, about 10% to about 40% byweight, about 10% to about 30% by weight, about 10% to about 25% byweight, about 10% to about 20% by weight, etc. For example, the amountof carbon obtained from the precursor can be about 1%, about 5%, about10% by weight, about 15% by weight, about 20% by weight, about 25% byweight, etc. from the precursor. When the amount of silicon is 90% orgreater by weight, the amount of carbon can be 10% or less by weight,e.g., about 0% or greater to about 3% by weight, about 0% or greater toabout 5% by weight, about 0% or greater to about 10% by weight, about 1%or greater to about 3% by weight, about 1% or greater to about 5% byweight, about 1% or greater to about 8% by weight, about 1% or greaterto about 10% by weight, about 5% or greater to about 10% by weight, etc.

The carbon from the precursor can be hard carbon (e.g., a glassycarbon). Hard carbon can be a carbon that does not convert into graphiteeven with heating in excess of 2800 degrees Celsius. Precursors thatmelt or flow during pyrolysis convert into soft carbons and/or graphitewith sufficient temperature and/or pressure. Hard carbon may be selectedsince soft carbon precursors may flow and soft carbons and graphite aremechanically weaker than hard carbons. Other possible hard carbonprecursors can include phenolic resins, epoxy resins, and other polymersthat have a very high melting point or are crosslinked. In someembodiments, the amount of hard carbon in the composite material canhave a value within a range of greater than 0% to about 95% by weightsuch as greater than 0% to about 90% by weight, greater than 0% to about80% by weight, greater than 0% to about 70% by weight, greater than 0%to about 60% by weight, greater than 0% to about 50% by weight, greaterthan 0% to about 40% by weight, greater than 0% to about 30% by weight,greater than 0% to about 20% by weight, about 1% to about 95% by weight,about 1% to about 90% by weight, about 1% to about 80% by weight, about1% to about 70% by weight, about 1% to about 60% by weight, about 1% toabout 50% by weight, about 1% to about 40% by weight, about 1% to about30% by weight, about 1% to about 20% by weight, about 5% to about 95% byweight, about 5% to about 90% by weight, about 5% to about 80% byweight, about 5% to about 70% by weight, about 5% to about 60% byweight, about 5% to about 50% by weight, about 5% to about 40% byweight, about 5% to about 30% by weight, about 5% to about 20% byweight, about 10% to about 95% by weight, about 10% to about 90% byweight, about 10% to about 80% by weight, about 10% to about 70% byweight, about 10% to about 60% by weight, about 10% to about 50% byweight, about 10% to about 40% by weight, about 10% to about 30% byweight, about 10% to about 25% by weight, about 10% to about 20% byweight, etc. In some embodiments, the amount of hard carbon in thecomposite material can be about 1% by weight, about 5% by weight, about10% by weight, about 20% by weight, about 30% by weight, about 40% byweight, about 50% by weight, or more than about 50% by weight. When theamount of silicon is 90% or greater by weight, the amount of hard carboncan be 10% or less by weight, e.g., about 0% or greater to about 3% byweight, about 0% or greater to about 5% by weight, about 0% or greaterto about 10% by weight, about 1% or greater to about 3% by weight, about1% or greater to about 5% by weight, about 1% or greater to about 8% byweight, about 1% or greater to about 10% by weight, about 5% or greaterto about 10% by weight, etc.

In certain embodiments, the hard carbon phase is substantiallyamorphous. In other embodiments, the hard carbon phase is substantiallycrystalline. In further embodiments, the hard carbon phase includesamorphous and crystalline carbon. The hard carbon phase can be a matrixphase in the composite material. The hard carbon can also be embedded inthe pores of the additives including silicon. The hard carbon may reactwith some of the additives to create some materials at interfaces. Forexample, there may be a silicon carbide layer between silicon particlesand the hard carbon.

In certain embodiments, graphite particles are added to the mixture.Advantageously, graphite can be an electrochemically active material inthe battery as well as an elastic deformable material that can respondto volume change of the silicon particles. Graphite is the preferredactive anode material for certain classes of lithium-ion batteriescurrently on the market because it has a low irreversible capacity.Additionally, graphite is softer than hard carbon and can better absorbthe volume expansion of silicon additives. In certain embodiments, theparticle size (e.g., a diameter or a largest dimension) of the graphiteparticles can be between about 0.5 microns and about 20 microns. All,substantially all, or at least some of the graphite particles maycomprise the particle size (e.g., diameter or largest dimension)described herein. In some embodiments, an average or median particlesize (e.g., diameter or largest dimension) of the graphite particles canbe between about 0.5 microns and about 20 microns. In some embodiments,the graphite particles may have a distribution of particle sizes. Forexample, at least about 95%, at least about 90%, at least about 85%, atleast about 80%, at least about 70%, or at least about 60% of theparticles may have the particle size described herein. In certainembodiments, the composite material can include graphite particles in anamount greater than 0% and less than about 80% by weight, including fromabout 40% to about 75% by weight, from about 5% to about 30% by weight,from about 5% to about 25% by weight, from about 5% to about 20% byweight, from about 5% to about 15% by weight, etc. When the amount ofsilicon is 90% or greater by weight, the amount of graphite can be 10%or less by weight, e.g., about 0% or greater to about 3% by weight,about 0% or greater to about 5% by weight, about 0% or greater to about10% by weight, about 1% or greater to about 3% by weight, about 1% orgreater to about 5% by weight, about 1% or greater to about 8% byweight, about 1% or greater to about 10% by weight, about 5% or greaterto about 10% by weight, etc.

In certain embodiments, conductive particles which may also beelectrochemically active are added to the mixture. Such particles canenable both a more electronically conductive composite as well as a moremechanically deformable composite capable of absorbing the largevolumetric change incurred during lithiation and de-lithiation. Incertain embodiments, a particle size (e.g., diameter or a largestdimension) of the conductive particles can be between about 10nanometers and about 7 micrometers. All, substantially all, or at leastsome of the conductive particles may comprise the particle size (e.g.,diameter or largest dimension) described herein. In some embodiments, anaverage or median particle size (e.g., diameter or largest dimension) ofthe conductive particles can be between about 10 nm and about 7micrometers. In some embodiments, the conductive particles may have adistribution of particle sizes. For example, at least about 95%, atleast about 90%, at least about 85%, at least about 80%, at least about70%, or at least about 60% of the particles may have the particle sizedescribed herein.

In certain embodiments, the mixture can include conductive particles inan amount greater than zero and up to about 80% by weight. In furtherembodiments, the composite material can include about 45% to about 80%by weight. The conductive particles can be conductive carbon includingcarbon blacks, carbon fibers, carbon nanofibers, carbon nanotubes, etc.Many carbons that are considered as conductive additives that are notelectrochemically active become active once pyrolysed in a polymermatrix. Alternatively, the conductive particles can be metals or alloysincluding copper, nickel, or stainless steel. When the amount of siliconis 90% or greater by weight, the amount of conductive particles can be10% or less by weight, e.g., about 0% or greater to about 3% by weight,about 0% or greater to about 5% by weight, about 0% or greater to about10% by weight, about 1% or greater to about 3% by weight, about 1% orgreater to about 5% by weight, about 1% or greater to about 8% byweight, about 1% or greater to about 10% by weight, about 5% or greaterto about 10% by weight, etc.

In certain embodiments, an electrode can include a composite materialdescribed herein. For example, a composite material can form aself-supported monolithic electrode. The pyrolysed carbon phase (e.g.,hard carbon phase) of the composite material can hold together andstructurally support the particles that were added to the mixture. Insome instances, the hard carbon phase can be a matrix phase (e.g.,glassy in nature) that is a substantially continuous phase. The siliconparticles can be homogeneously distributed throughout the hard carbon.In certain embodiments, the self-supported monolithic electrode does notinclude a separate substrate, collector layer, and/or other supportivestructures. In some embodiments, the composite material and/or electrodedoes not include a polymer beyond trace amounts that remain afterpyrolysis of the precursor. In further embodiments, the compositematerial and/or electrode does not include a non-electrically conductivebinder.

The composite material may also include porosity, such as about 1% toabout 70% or about 5% to about 50% by volume porosity. For example, theporosity can be about 5% to about 40% by volume porosity.

In some embodiments, the composite material can be attached to a currentcollector. For example, the composite material can be laminated on acurrent collector using an electrode attachment substance (e.g., apolymer adhesive). In some embodiments, the composite material may alsobe formed into a powder. For example, the composite material can beground into a powder. The composite material powder can be used as anactive material for an electrode. For example, the composite materialpowder can be deposited on a collector in a manner similar to making aconventional electrode structure, as known in the industry.

In certain embodiments, an electrode in an electrochemical device suchas a battery or electrochemical cell can include a composite materialdescribed herein. The composite material can be substantiallyelectrochemically active. The composite material can be used for theanode and/or cathode. The electrochemical device can includeelectrolyte, and can be a battery. In certain embodiments, the batteryis a lithium ion battery. In further embodiments, the battery is asecondary battery, or in other embodiments, the battery is a primarybattery.

Furthermore, the full capacity of the composite material may not beutilized during use of battery to improve life of the battery (e.g.,number charge and discharge cycles before the battery fails or theperformance of the battery decreases below a usability level). Forexample, a composite material with about 70% by weight siliconparticles, about 20% by weight carbon from a precursor, and about 10% byweight graphite may have a maximum gravimetric capacity of about 2000mAh/g, while the composite material may only be used up to angravimetric capacity of about 550 to about 850 mAh/g. Although, themaximum gravimetric capacity of the composite material may not beutilized, using the composite material at a lower capacity can stillachieve a higher capacity than certain lithium ion batteries. In certainembodiments, the composite material is used or only used at agravimetric capacity below about 70% of the composite material's maximumgravimetric capacity. For example, the composite material is not used ata gravimetric capacity above about 70% of the composite material'smaximum gravimetric capacity. In further embodiments, the compositematerial is used or only used at a gravimetric capacity below about 50%of the composite material's maximum gravimetric capacity or below about30% of the composite material's maximum gravimetric capacity.

EXAMPLES Example 1

In one example, a slurry was prepared from silicon powder, graphitepowder, polyamic acid, and N-methyl-2-pyrrolidone. The slurry was coatedonto a PET substrate and dried, with a residual solvent content of −17%and a solid loading of 3.8 mg/cm². The green film was peeled from thePET substrate, cut into pieces, and stacked in a graphite supportfixture with an interleaf of polyethylene film (for example, as shown inFIG. 4). The assembly was thermally treated under argon, with a ramprate of 5° C. per minute up to 1175° C., and a 1 hour dwell at 1175° C.The polyethylene film decomposed, leaving minimal carbon residue (<1%),and the layers of silicon composite film were easily separated. Thefilms were laminated to PAI-coated copper foil to form silicon compositeelectrodes (e.g., anodes), assembled into cells, and electrochemicallytested.

Example 2

In another example, a slurry was prepared from silicon powder, graphitepowder, polyamic acid, and N-methyl-2-pyrrolidone. The slurry was coatedonto a polypropylene substrate and dried, with a residual solventcontent of −20% and a solid loading of 3.8 mg/cm². The resulting bilayerfilm was cut into pieces and stacked in a graphite support fixture (forexample, as shown in FIG. 5). The assembly was thermally treated underargon, with a ramp rate of 5° C. per minute up to 1175° C., and a 1 hourdwell at 1175° C. The polypropylene film decomposed, leaving minimalcarbon residue (<1%), and the layers of silicon composite film wereeasily separated. The films were laminated to PAI-coated copper foil toform silicon composite electrodes (e.g., anodes), assembled into cells,and electrochemically tested.

FIG. 6 shows a graph of the capacity retention versus cycle index ofelectrochemical cells with electrodes prepared with sacrificialsubstrates using a method as described in Example 1 (e.g., shown in FIG.4) compared with electrochemical cells with electrodes prepared withoutsacrificial substrates. The silicon composite electrodes were composedof 80% silicon, 5% graphite, and 15% carbon, with a loading of 3.8mg/cm², laminated to a 15 μm thick copper foil coated with a 0.4 mg/cm²polyamide-imide film. They were assembled into cells with cathodescomposed of 97% lithium cobalt oxide, 1% carbon black, 2% PVdF coated on15 μm thick aluminum foil with a loading of 28 mg/cm². The electrolytewas composed of 1.2M LiPF₆ in FEC/EMC/DEC (2/4/4 vol %)+2 wt % propanesultone+2 wt % adiponitrile. The 620 mAh cells were tested with a2C-charge rate to 4.3V and a 0.5 C-discharge rate to 3.3V, with adischarge to 2.75V every 50 cycles.

FIG. 7 shows a graph of the capacity retention versus cycle index ofelectrochemical cells with electrodes prepared with sacrificialsubstrates using a method as described in Example 2 (e.g., shown in FIG.5) compared with electrochemical cells with electrodes prepared withoutsacrificial substrates. The silicon composite electrodes were composedof 80% silicon, 5% graphite, and 15% carbon, with a loading of 3.8mg/cm², laminated to a 15 μm thick copper foil coated with a 0.4 mg/cm²polyamide-imide film. They were assembled into cells with cathodescomposed of 92% lithium nickel cobalt manganese oxide, 4% carbon black,4% PVdF coated on 15 μm thick aluminum foil with a loading of 23 mg/cm².The electrolyte was composed of 1.2M LiPF₆ in FEC/EMC (3/7 wt %). The565 mAh cells were tested with a 4C-charge rate to 4.2V and a 0.5C-discharge rate to 3.1V, with a discharge to 3V every 100 cycles.

As shown in FIGS. 6 and 7, cells made with electrodes produced via themethods described herein had improved capacity retention compared tocells made with electrodes produced via a method without usingsacrificial substrates.

Without being bound by theory, the reason for the performanceimprovement may be due to one or more of the following reasons: (1) anincrease in carbon content at the surface of the electrode (potentiallyimproving electrical conductivity), (2) an alteration of the pyrolysisconditions (the surface carbon may react more readily with trace oxygen,reducing or preventing degradation of the bulk carbon matrix or furtheroxidation of the silicon surface), (3) mechanical effects due to thesacrificial film being gasified and also reactions with the gassescreated by the sacrificial layer, and/or (4) the sacrificial layer couldaffect the atmosphere around the electrode being pyrolysed to change thepyrolysis equilibrium during the heat treatment.

The use of sacrificial substrates provides several benefits in themanufacture of silicon composite electrodes. For example, usingsacrificial substrates can reduce the formulation and processrestrictions required in order to peel the green film and handle itunsupported. This can allow for a higher silicon content (and thereforehigher theoretical energy density), and also the use of different carbonprecursors (which can reduce cost by using a lower molecular weight,less flexible resin for example). It can also reduce cost by reducingthe drying requirements of the material prior to thermal treatment, asthe layers of green material can be separated with sacrificial materialso that the films are not adjacent and do not bond to each other.

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. A method of forming a composite material film, the method comprising: providing a layer comprising a carbon precursor and silicon particles on a sacrificial substrate, wherein the layer comprises a mixture comprising a solvent; drying the mixture on the sacrificial substrate prior to pyrolysing, wherein the dried mixture comprises from about 10% to about 30% of the solvent; forming the dried mixture on the sacrificial substrate into a plurality of dried mixtures on sacrificial substrates, and stacking the plurality prior to pyrolyzing; and pyrolysing the carbon precursor to convert the precursor into one or more types of carbon phases to form the composite material film, whereby the sacrificial substrate has a char yield of about 10% or less.
 2. The method of claim 1, wherein the sacrificial substrate has a char yield of about 7% or less.
 3. The method of claim 2, wherein the sacrificial substrate has a char yield of about 5% or less.
 4. The method of claim 3, wherein the sacrificial substrate has a char yield of about 3% or less.
 5. The method of claim 4, wherein the sacrificial substrate has a char yield of about 1% or less.
 6. The method of claim 5, wherein the sacrificial substrate has a char yield of about 0%.
 7. The method of claim 1, wherein the sacrificial substrate comprises polymethylpentene (PMP), acetal copolymer, acrylonitrile butadiene styrene (ABS), paraffin wax, polyethylene oxide, polyethylene, polypropylene, poly(propylene carbonate), cellulose acetate, or a combination thereof.
 8. The method of claim 7, wherein the sacrificial substrate comprises polyethylene, polypropylene, poly(propylene carbonate), polymethylpentene, or a combination thereof.
 9. The method of claim 1, wherein the sacrificial substrate is insoluble in the solvent.
 10. The method of claim 1, wherein the solvent comprises N-Methylpyrrolidone (NMP).
 11. The method of claim 1, wherein the solvent comprises water.
 12. The method of claim 11, wherein the carbon precursor comprises a water soluble polymer.
 13. The method of claim 1, wherein the composite material film comprises the one or more types of carbon phases at greater than 0% to about 20% by weight.
 14. The method of claim 1, wherein the composite material film comprises the silicon particles at greater than about 50% to about 99% by weight.
 15. The method of claim 1, wherein the composite material film further comprises graphite particles.
 16. The method of claim 1, wherein the composite material film is substantially electrochemically active.
 17. The method of claim 1, wherein pyrolysing forms the composite material film as a self-supported structure.
 18. The method according to claim 1, comprising: providing a first electrode, wherein providing the first electrode comprises providing the composite material film formed by the method of claim 1; providing a second electrode; and providing an electrolyte.
 19. The method of claim 18, wherein providing the first electrode comprises providing the composite material film as a self-supported electrode.
 20. The method of claim 18, wherein providing the first electrode comprises providing the composite material film on a current collector.
 21. The method of claim 18, wherein the first electrode is an anode and the second electrode is a cathode.
 22. The method of claim 18, comprising: forming an electrochemical device; wherein forming the electrochemical device includes providing the first electrode, providing the second electrode, and providing the electrolyte; and wherein the electrochemical device is a battery. 